Evaluating the Effects of Aerial Fire Retardants on Soil Chemistry: A Bridge Fire Case Study

As climate change intensifies the severity and frequency of wildfires, fire suppression strategies have grown increasingly reliant on aerially deployed fire retardants. While there are regulations designed to minimize risks associated with fire retardants, the effects of accessory chemicals on soil chemistry remain under-researched. This study aimed to investigate the soil chemistry implications of the aerial fire-retardant Phos-Chek in the wake of the 2024 Bridge Fire in the San Gabriel Mountains, California. The goal was to determine if accessory chemicals, specifically heavy metals, found in isolated Phos-Chek samples would be present in treated soils. Soil samples were collected from three locations in the Angeles National Forest. The first, Ice House Canyon, was primarily unburned; Baldy Mountain Road was partially burned and borders Baldy Village, and Glendora Ridge Road was fully burned. At each location, surface soils to a depth of approximately 15 cm were collected, air-dried, sieved at 2 mm, and prepared for further analysis. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and portable X-Ray Florescence (PXRF) analyses were conducted to determine trace and major elemental concentrations in the soils. The hydrometer method was used to determine the soil textures and the pH for each sample was also measured. Previous studies found that concentrations of Cr, Cd, As, Pb, Cu, V, Mn, Sb, and Tl were above regulatory levels in Phos-Chek. The results of this study showed that across all samples, the concentrations of these heavy metals were either below levels of detection or only slightly elevated from baseline concentrations. Further studies are essential to determine the residence time of introduced accessory chemicals from fire retardants to understand implications to soil health.